In recent years, the development of a ship driven by electrically driving a motor has been advanced in view of suppressing a depletion of fuel resources, such as a gasoline, and an environmental deterioration due to exhaust gas. Specifically, a superconducting motor disclosed in JP-A-6-006907 (Patent Document 1) may be employed, thereby eliminating copper loss in a superconducting coil and achieving high efficiency. Additionally, a miniaturization of the motor itself and a high-power output can be achieved.
Meanwhile, when driving a superconducting motor, a superconducting coil is required to be cooled to an ultra-low temperature (e.g., 77 kelvins). Thus, means for cooling is especially important, and a simple and efficient cooling structure is being required.
Particularly, when cooling a superconducting coil attached to a rotor disposed at a center of a motor, it is inefficient to cool it from outside the motor. Therefore, it is difficult to sufficiently cool the superconducting coil in such a case.
JP-A-2002-58207 (Patent Document 2) provides a structure in which a hollow portion is formed through a rotating shaft of a motor and a refrigerant is passed through the hollow portion to cool a coil attached to a rotor. This structure enables an efficient cooling of the coil attached to the rotor to a necessary temperature even in a case where the coil attached to the rotor is a superconducting coil.
However, in the structure disclosed in Patent Document 2, a center of the rotating shaft needs to be drilled to form the hollow portion. Particularly, in a case of a large motor, such as a series-coupled synchronous type motor used to drive a ship, it is difficult to form a long hollow portion through a long rotating shaft.
Patent Document 1: JP-A-6-6907
Patent Document 2: JP-A-2002-58207